In prior production methods, the wafer cleaning of the slurry residues after the wire-sawing and the wafer texturing are completed in two different process chains. Both processes are implemented operationally by wafer producers and solar cell producers. Slurry wafer cleaning is effected immediately after the wire-sawing by the wafer producers. It essentially comprises removing contamination applied on the wafer surface by the wire-sawing. There is included therein above all the abrasion of silicon and also of the components of the wire and the grinding agent and adhesive used (slurry). The cleaned wafers, after leaving the cleaning unit, have a more or less intensive crystalline damage to the surface which must be removed in a further process step.
The texturing of the wafer surface, if it has been implemented at all, falls within the remit of solar cell producers. Preceding this, during production of highly efficient solar cells, is a complex three-part subsequent cleaning step which follows the wafer subsequent cleaning in the chip industry. This RCA cleaning (D. C. Burkman, D. Deal, D. C. Grant, C. A. Peterson: “Aqueous Cleaning Processes in Handbook of Semiconductor Wafer Cleaning Technology, Science”, Technology and Applications, East Windsor, N.J., USA (1993) pp. 111-129) comprises:    1.) Oxide etching with HF/HNO3;    2.) SC-1 cleaning with H2O/NH4OH/H2O2 with subsequent HF dip;    3.) SC-2 cleaning with (H2O/HCl/H2O2) which removes the contamination and residual metallic and organic contaminants, which are applied on the wafer surface by the transport, after the pre-cleaning by the wafer producers (Figure 1a).
After each chemical treatment, the wafers pass through another rinsing step with highly pure, deionised water before moving to the next etching solution. In this way, a total number of tanks of approx. 8 is the rule solely for the RCA cleaning (O. Doll: “Komplexbildner für alkalische Reinigungslösungen von Halbleitersilicium-Oberflächen: Aspekte ihrer Wirksamkeit and ihrer chemischen Stabilität”, (Complex formers for alkaline cleaning solutions of semiconductor silicon surfaces: aspects of their efficacy and their chemical stability), PhD Thesis, Frankfurt/Main (2005)) with small deviations by the various producers.
In the case of commercial solar cells, the wafers cleaned by the wafer producers of the slurry move directly into the texturing process which, in the case of basic texturing according to the prior state of production of commercial solar cells, is implemented on an industrial scale in a batch unit and serves simultaneously also as damage etch. However, it is also possible that the damage etch is implemented as a separate process step before the texturing. After applying the texturing, the wafers are subjected to an HCl and HF dip for respectively up to one minute before the emitter diffusion is implemented (Figure 1b). The HCl dip is intended to neutralise the residues of KOH solution remaining on the wafer surfaces and to stop their further chemical attack on the wafer surface. Likewise, the metallic impurities deposited by the alkaline etching solution in addition on the silicon surface, above all the poorly soluble hydroxides, precipitated from the basic solution, of the trivalent metal ions Fe3+ and Al3+, are intended to be brought again into solution. The HF dip is intended to ensure removal of the native oxide present on the wafers after the texturing step.
The wafer cleaning has been performed to date by two method routes: either in a batch or in an inline process, respectively two units, one for the slurry cleaning and one for the texturing, being used. To date a batch process has been the industrial standard for wafer texturing. Also different chemical systems are thereby used for cleaning and texturing: only weakly basic agents (pH 8-9) which contain cleaning surfactants for the cleaning, and strongly basic agents (pH>13) which contain texture surfactants for the texturing. Cleaning surfactants and texturing surfactants are not identical in the case of previous solar cell processing. In both cases, the pH of the agents is regulated via the addition of alkali hydroxides (generally NaOH in the cleaning and KOH in the texturing).
In the case of the batch cleaning process, the contaminated wafers pass through chemical tanks of a different composition in defined portions. Generally, aqueous alkaline agents based on potassium- or sodium hydroxide with different surfactant additives serve as cleaning agents. Chemical and ultrapure water cascades make use of the dilution effect as an essentially physical principle during cleaning, which dilution effect results when the wafers move through a large number of tanks. The process time for a batch, comprising all those partial steps from wafer separation after the wire-sawing up to the finished cleaned wafer, is generally more than one hour here.
Inline cleaning ensures a tighter temporal course in which the wafers are conveyed over a roller system in the manner of a conveyor belt, said wafers being subjected in the various portions of the cleaning bank, which are not delimited from each other strictly, to variable chemical and physical conditions. Inline cleaning, unlike batch cleaning, is a continuous process. It enables continuous exchange of the cleaning agents in parallel for cleaning without interruption of the process and improved agent control. From a chemical point of view, similar cleaning agents are used here as in batch cleaning.
For texturing of the surface of monocrystalline silicon wafers, generally an alkaline agent comprising potassium hydroxide and 2-propanol is used in the case of solar cell production. In order to ensure sufficient etching removal for removing the sawing damage, etching times in the range between 15 and 25 minutes are normal, for which reason production plants are used exclusively in the batch process. In the case of texturing, the anisotropy of alkaline etching agents is used in the etching behaviour of different crystal directions in the silicon in order to produce so-called “randomly distributed pyramids”, “random pyramids”. As further alkaline etching agents in addition to KOH, furthermore sodium hydroxide, tetramethylammoniumhydroxide and ethylenediaminepyrocatechol are mentioned, the organic etching agents mentioned here differing above all from both inorganic ones in a longer processing time, however the trade-off is they have no metallic cations. Potassium hydroxide is preferred relative to sodium hydroxide in the case of the inorganic etching agents, because the sodium ions remaining on the surface after the texturing can act, because of their higher diffusion rate relative to the larger potassium ions, as mobile charge carriers in the surface oxide of the silicon wafers. This has a negative effect on the electrical properties of above all oxide-passivated silicon substrates.